In recent years, various methods have been studied to form an electric conductor by forming a pattern employing an ink comprising a dispersion having fine particles of a metal such as copper dispersed in a liquid, followed by a heat treatment so that the metal fine particles are mutually sintered. Such methods may, for example, be a method of carrying out formation, repair or the like of a circuit pattern such as printed wiring by means of an ink jet printing method as disclosed in JP-A-2002-324966, a method of joining metals which replaces conventional soldering, as disclosed in JP-2002-126869, and a method of forming an electrically conductive metal film capable of replacing a plated film in the field of electronic materials, as disclosed in JP-A-2002-334618.
The above-described methods employ known nature called surface melting of metal particles (“J. Sol-Gel Science and Technology” the Netherlands, Kluwer Academic Publishers, 2001, vol. 22, pages 151 to 166). It is generally known that the surface melting of metal particles takes place due to abnormal lattice vibration of atoms at the surface of the particles, and that the smaller the particle diameter and the higher the proportion of surface atoms, the more the surface melting temperature decreases. For example, in a case of copper, it is known that bulk copper has a melting point of 1,083° C., whereas fine particles having diameters of about 10 nm undergo the surface melting at a temperature of about 150° C. Since the surface melting depends on the particle diameter of the metal particles, it takes place even in an association state when individual fine metal particle has a predetermined particle diameter, unless particles are completely bound.
However, in general, metals are likely to be oxidized except for noble metals, and fine particles having particle diameters of at most 100 nm, which have large surface areas, are remarkably influenced by the surface oxidation, and the surface of the fine metal particles are oxidized also when an electric conductor is to be formed, whereby an electrical conductivity is less likely to be obtained.